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Michael Phelps welcomes son, Boomer Robert Phelps, with fiancée Nicole Johnson

The champion swimmer welcomed a son with fiancee Nicole Johnson on Thursday.
/ Source: TODAY

Michael Phelps has 18 Olympic gold medals, but the "best feeling" he's ever felt? Becoming a dad!

The champion swimmer welcomed a son, Boomer Robert Phelps, with fiancée Nicole Johnson on Thursday.

Phelps shared the news on Instagram Saturday with a photo of little boomer and Johnson. He wrote in the caption: "Healthy and happy!!! Best feeling I have ever felt in my life!!!"

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Phelps announced that he and Johnson, who was Miss California in 2010, were expecting in November, with an Instagram appropriately taken on a pool deck.

"We found out 8 weeks ago we were expecting," the swimmer said in the caption.

In February, the couple announced their engagement with a sweet, snowy photo.

Little Boomer arrived just months before Phelps plans to take on his fifth (and final) Olympic Games. In an interview April with TODAY's Matt Lauer, Phelps offered up how he hopes people describe his performance in Rio:

"It would say something along the lines of, I finally gave 100 percent," he said. "Or I'm happy to turn the page to the next chapter. I'm happy to move on from my swimming career and go out how I wanted to."